This invention relates generally to heat shrinkable, relatively gas impermeable thermoplastic multilayer films which can be used in the form of a hermetically sealed bag for packaging meat products, as for example fresh red meat or cook-in systems wherein the packaged meat is submerged in heated water for cooking. Under these conditions the bag film is shrinkable against the meat and retains its integrity.
As used herein, "cook-in" refers to packaging material structurally capable of withstanding exposure to long and slow cooking conditions while containing a food product, for example submersion in water at 68.degree.-85.degree. C. (115.degree.-185.degree. F.) for 4-6 hours. Cook-in packaged foods are essentially pre-packaged, pre-cooked foods which may be directly transferred to the consumer in this form. These types of foods may be consumed with or without warming. Cook-in packaging materials maintain seal integrity and in the case of multilayer films, are delamination resistant. Cook-in films must also be heat shrinkable under cook-in conditions so as to form a tightly fitting package.
Some films are suitable for holding meats such as ham which is cooked at temperatures up to about 82.degree. C. (180.degree. F.), but are not always suitable for holding meats such as beef which are sometimes cooked at higher temperatures up to about 92.degree. C. (200.degree. F.), hereinafter referred to as the "high temperature cooking range". For example, Surlyn 1650 (manufactured by the DuPont Company) may be used as the food-contacting layer of a multilayer cook-in film for ham, but would not be satisfactory for the high temperature cook-in range because its melting point is 94.degree. C. (205.degree. F.). Accordingly, cook-in films suitable for the high temperature cook-in range require a food-contacting layer having a melting point of at least 100.degree. C. (212.degree. F.).
Cook-in films for beef in the high temperature cooking range should have an inner surface which does not adhere to the meat. Beef is relatively dry and has a fiber structure which would be retained by an adhering film surface causing the disrupted surface of the cooked product to be asthetically unpleasing to the consumer. This is to be contrasted from ham cook-in wherein the film inner surface should adhere to the ham outer surface to retain the juices within the relatively moist, ham product.
Another requirement for certain multilayer clear films is that they have good optical clarity, i.e. gloss of at least about 55% and haze below about 15%.
There are numerous other requirements for a high temperature shrink film, including: delamination resistance, low oxygen permeability, and heat shrinkability representing about 20-45% biaxial shrinkage at about 90.degree. C. (194.degree. F.).
In U.S. Pat. No. 4,606,922 the prior art has described an irradiated multilayer food casing having an outer nylon layer over one or more interior layers and having a Surlyn food contact inner surface, for example, the structure nylon (outer)/adhesive/Surlyn (inner). Nylon is described as providing an oxygen barrier to impede the inward diversion of oxygen and to impart relatively high stuffing strength to the casing. However, this film is not shrinkable and must be stuffed with a flowable meat product to insure sufficient film contact. Also, the nylon outer layer loses its effectiveness as an oxygen barrier because of moisture uptake from the atmosphere. Finally, Surlyn adheres to meat so would be unsuitable as the inner surface of a beef cook-in film.
U.S. Pat. No. 4,606,922 also describes a four layer shrinkable film comprising EVA/saran/EVA/ionomer. However, the necessary irradiation has a degrading effect on the vinyl chloride-vinylidene copolymer (saran).
Another commercially employed cook-in film is the type described in Oberle et al U.S. Pat. No. 4,469,742, comprising six layers irradiated to dosage of preferably at least 6 MR. The central barrier layer is hydrolyzed ethylene-vinyl acetate copolymer (EVOH), chosen because of its higher softening point and superior oxygen barrier properties as compared to the saran-type barrier layer commonly used in multilayer films for ambient temperature applications. On each side of the barrier layer is an adhesive layer such as chemically modified polyethylene, eg. Plexar. On the outer side of each adhesive layer is a shrink layer such as ethylene-vinyl acetate copolymer (EVA), and the outside (abuse) layer is also the EVA type having a vinyl acetate content of about 5-12 wt. %. The innermost (heat sealing) layer may for example be an ethylene-propylene random copolymer. Processing of a six layer film requires a complex die and careful resin selection within a limited number of possibilities.
It will be apparent from the foregoing that these prior art cook-in films are complex both in terms of multiple materials and sophisticated manufacturing techniques, and some are not heat shrinkable.
A possible solution to the foregoing problems is a four layer film with EVOH as the barrier layer, but tests have shown that such prior art films with even only three layers, i.e., no inner layer designed for high temperature cook-in contact with beef, either would not biorient, would tend to delaminate during cooking or have poor optical properties or have unacceptably low shrink value for cook-in applications, or a combination of these problems. Certain of these problems may be overcome by the use of EVOH-polyamide blend core layers, but multilayer films of this type with EVA inner and outer layers have typically been found to delaminate under cook-in conditions and in any event would not withstand high temperature beef cook-in.
A possible approach to this delamination problem is the use of adhesives, but adhesives suitable for EVOH would not be expected to biorient or in heavier gauges might be expected to be hazy or they might not provide the required shrinkage.
One object of this invention is to provide an improved high temperature cooking range multilayer plastic film suitable for fresh red meat and cook-in systems which has less than six layers including a nonadhering inner surface for meat contact.
Another object of this invention is to provide a less-than-six layer high temperature cooking range plastic film which has good optical clarity and a nonadhering meat contact surface.
Still another object of this invention is to provide a less-than-six layer high temperature cooking range type film having high shrink, good optical clarity, a nonadhering meat contact surface and no blocking problem.
A further object of this invention is to provide such a less-than-six layer high temperature cooking range type film which does not require additives such as processing aids and does not require high irradiation dosage level on the order of 6 MR to realize high strength.
Other objects and advantages will be apparent from the ensuring disclosure and appended claims.